Signal system using phase modulation



May 30, 1944. w. P. PLACE I SIGNAL SYSTEM USING PHASE MODULATION Filed Jafi. 12, 1942 lave.

H y o E 5 M W 0 N n my P W H Patented May 30, 1944 SIGNAL SYSTEM USING PHASE MODULATION I Willard P. Place, Penn Township, Allegheny County, Pa., assignor to The Union-Switch & "Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application January 12, 1942, Serial No. 426,376 5Claims. (c1.177-s5s) 'My invention relates to signal systems using phase modulation, and more particularly to railway train-signal systems using phase modulations I U i I =--Manysignal systems use energy that is coded by' periodically interrupting thecircuit according to a desired code. For example, certain types of code cab signal systems forrailway'trainsuse alternating currentthat is coded at any one of several different codes'by interrupting the circuit at any one of several different rates, such as 180, 120 and 75 times per minute. That is,' time is divided into successive code cycles each of which consists of an onperiod during which current flows and an off period during which no current flows, these two' code periods being preferably of substantially equal durations. Broadly speaking, such coding of alternating current is amplitude modulation of the current, 100 per cent modulation being effected. Such coded current is supplied to a control circuit atone point for' actuating receiving apparatus including a decoder coupled to the circuit at some remote point, the decoder in turn governing a signal device in 'accordance with the code transmitted. In a railway train cab signal system, the control circuit is a track circuit including the rails and train carried receiving-apparatus has its input side coupled to the rails and is energized in response to the electromotive force picked up from the rails due to the coded current flowing therein, such receiving apparatus includ iri'g decoding means responsive to the different code rates of such received electromotive force for selectively governing the position of a mul tiple position cab sig'nalor other train control device.

One of the limiting factors which determine the amount of power that must be supplied to a control circuit for operating a signal is the amount of interference. Again referring to-present day railway train code signal systems, a limiting factorwhichdetermines the smallest wayside power that'can beused to operate a cab signal over a given length of track is-the'amount of stray magnetic'fields created by outside circuits in the vicinity of the track, and by the so-called magnetized spots in the rails. It has been found that such interference in a cab signal system is more pronounced when the control or carrier current is of a low-frequency. On the other hand, the attenuation caused by the track rails increases when alternating current of a relatively high frequency is used, and furthermore the degreeofbroken' rail protection becomes less as 66 the frequency of the control current is increased. The problem of broken rail protection sets an upper limit of about 20 cycles per second forthe control current if relatively long track circuits of .the order of 11,000 feetare to be used, such long track circuits being desirable because of the present day high train speeds. Since energy is picked up inductively, the current level must be raised for effective operation of the usual train carried receiving apparatus when current of a frequency lowenoughto insure ample broken rail protection is used, and thus any power saving must come about through reducing the effects of the interference caused by stray magnetic fields. Power saving is desirable because in many cases the alternating current is obtained from a battery and a tuned vibrator. Phase modulation has the advantage over amplitude modulation in that a power gain at the transmitter is obtained: That is, for a given transmitter, the power supplied to the control circuit is increased when phase modulation rather than amplitude. modulation is used. Experience of investigators the radio field indicates that with phase modulation a four to one advantage in the power handling capacity of the transmitter isobtained. It has also been found that in equal carrier noise ratios, the peak signal noise ratios obtained by phase and amplitude modulations areabout equal. Hence a gain in power obtained by phase modulation results in better overcoming the usual interference.

A wave is phase modulated when its instantaneous phase is deviated fromfthe position it would have taken if the modulation werenot present. Such phase shift may be introduced, for example, by passing the wave through a circuit element or network which imparts a time delay to the wave so that the wave at the output of the network has a phase which is different from that at the input.

In view of these circumstances in signal systems, a feature of my invention is the provision of a novel and improved organization of apparatus for a signal system using phase modulation.-

1 .Anotherfeature of my invention is the provision of a novel and improved organization of apparatusfor railway train cab signal systems using phase modulation. 1

Still another feature of my invention'ls the provision of improved apparatus for railway train cab signal systemswherewith saving of power at the" transmitter is obtained, better apparatus that comprises a source of carrier cur-" rent, a phase modulating element and a coding device. cyclic operation which is divided into two substantially equal periods, a first'contact being closed during the first period of each cycle, and

a second contact being closed during the second period of each cycle. The source of current is connected to the control or. transmitting circuit over two alternative circuit paths.

path includes the first contact of the vice but nothing that will shift the-phase of the current as, supplied by the current source. The other circuit path includes thesecond contactof the coding device and the phase modulating, element which element acts to shift the phase a predetermined degree. Thu during the first code period of each code cycle non-modulated carrier current is supplied to the control circuit, and during, each second code period of each code cycle, phase modulated carrier current is supplied to the control circuit. Preferably, a plurality of such coding devices are provided, each of which. is actuated at a particular cycle. In other words, each coding deviceis operated, at the particular code rate. A multiple position control means selects the particular coding device that is to be effective. I 4 The receiving apparatus includes receiving means coupled to the control circuit to receive an ele'ctromo-tive force. of a frequency, phase modulation and code corresponding to that of the current supplied by the transmitting apparatus. The receiving means is preferably connectedto an amplifier to amplify the received en'ergy, but if the received energy is of relatively high energy level such amplifier. may not be required. A phase demodulator is connected to the "re eiv n means or to the" output of the amplifier, if used, 'such' demodulator being effective to provide at itsoutput 'an'electromotive' forcewhich is varied inaccordance to the code rate of the received energy; This output electromotivefforce of the demodulator is used to operate a code following relayrsuchrelay beingoperated at a rate corresponding to the code rate of the received energy.

The code following relay governs the supply of currentto a decoding means-selectively responsiveto the code rate or frequency of the current impulses supplied thereto, and such decoding means in turn governs a multiple position signal device so that the position of the signal device is made to correspond to the code rate of the transmitted energy.

As an aid to the understanding of my invention reference may be had to the accompanying draw ing Which is a diagrammaticylew showing one form of apparatus embodying my invention when used fora code cab signal system for a railway train. It is to be understood thatI do not limit my invention to cab signal systems for railway trains and this one applicationserves to illustrate the many places where such apparatus is useful and at the same time serves to show the peculiar utility of such apparatus when used for control of a railway train cab signal.

One such coding d'e track section D-E may be of any suitable length Such coding device is actuated in a as required. For example, section DE may be a relatively long section of the order of 11,000 feet, which length of track section is desirable v where present day high train speeds prevail.

The section D-E is provided with a track circuit which includes a source of current connected across the rails at one end of the section and a track relay connected across the rails at the other end of the section. The means for supplying current to the track circuit of section D' E comprises a source of alternating current of a preselected frequency, a code transmitting means, a phase modulating means and a traflic controlled relay. The immediate source of alternating current is a transformer TD whose primary winding 2 is connected to a generator, not shown, and which generator delivers alternating current of a preselected frequency. For example, the alternating current may be obtained from a transmission line or from a tuned vibrator operated from a local battery. The frequency of the current supplied by such generator may be of the order of 100 cycles per second which is the frequency generally used in present day cab signal systems, and. which frequency provides satisfactory broken rail protection when track sections of the order of 5,000 feet in length are used. However, the frequency of the current supplied to transformer TD may be of the order of 20 cycles per second so that satisfactory broken rail protection for track sections of the order of 11,000 feet in length is provided, or the frequency may be of some intermediate frequency, such as 30 or 40 cycles per second.

The code transmitting means is here shown as a coder CD which may be of any one of several well-known forms, and it is sufficient for this application to point out that coder CD is provided with two code contact members 180 and which are actuated in a cyclic operation as long as an operating winding 8 of the coder CD is supplied with current from any suitable source. For example, contact member I may be operated to alternately engage contacts 180a and I8Ilb at the code rate of 180 times per min- .ute, and contact member 75 may be operated to alternately engage contacts 15a and 751) at the code rate of 75 times per minute.

The phase modulating means may take difierent forms and as here shown it is a circuit network unit PM that includes an inductor 3 and a. condenser 4 so proportioned and disposed that alternating current supplied to the input terminals'5 and 6 of the unit PM from secondary winding 1 of transformer TD is delayed to an extent suflicient that the phase of the alternating current, appearing at the output terminals 9 and l 0 of unit PM is different from that of the phase of the current applied to input terminals 5 and 6. The phase shift would be predetermined by the proportioning of the parts of the modulatin unit'PM, and may be for example a phase shift of the order of degrees.

Traffic controlled relay HD is governed according to traffic conditions in advance of section D-E and as here contemplated relay HDls PM, inductor 3, output terminal modulation is supplied to the rails.

picked up closing front contact II when the section in advance of section DE is unoccupied and clear trafilc conditions prevail, and relay HD is released closing back contact l2 when such section in advance is occupied andapproach trafiic Current supplied to the rails at the exit end D of section DE for transformer TD is phase modulated and coded according to the position. of trafiic controlled relay HD. To be explicit, when relay ED is picked up to close front contact II,

secondary winding .1 of transformer TD is connected across the rails over two alternative paths controlled by code contactmember I80 of coder CD. One such path can be traced from the top terminal of secondary winding 1 over wire IS,

a resistor I4, contact I80-I80a, front contact H N of relay HD, Wire I5 to rail lb, thence to rail Id through a track relay to be referred to later, or

through the train shunt of any train occupying thesection, and from rail Ia over wire I6 to the lower terminal of secondary winding I, wire It passing between terminals I and 6 of unit PM, as will be apparent from an inspection of the drawing. The other path. includes top terminal of secondary winding 1, inputterminal of unit I801) of coder CD, front contact II of rela HD, and thence as traced for the firstpath back to the lower terminal of secondary Winding I, condenser 4 being connected across terminals 9 and I0. It is clear that current of the original phase is supplied to the rails over the first traced path closed atcontact I80I80a of coder CD, and current phase shifted as predetermined by the proportioning of the unit PM is supplied to the rails over the second path closed at contact Its- I801) of the coder. In other words, during the first half period of the cyclic operation of code contact member I80 of coder CD, non-modulated current is supplied to the rails, and each second code period of the cyclic operation of contact member I00, current of a predetermined phase When control relay HD is released, closing back contact I2, transformer TD is connected across the rails over two alternative paths governed by code contact member 15.: A first one of these two alternative paths extends from the top terminal of secondarywinding I of transformerTD over wire I3, resistor I4, contact IE-a of coder CD, back contact I2 of rela HD, .wire Iiito rail lb, thence through the track relay or train shunt to rail Ia and from rail Ia over wire I6 to the lower terminal of secondary winding 'I. The other path of these two alternative paths includes modulating unit PM and contact 15-452) ofcoder CD as will be apparent from an inspection of the drawing. Consequently during each 'first half period of the cyclic operation of code contact member 15 and contact IS-15a is closed,

non-modulated current issupplied to the track rails, and during each second half period of the cyclic operation of contact member'IS and contact 15I5b is closed current of the preselected phase modulation is supplied to the rails.

To sum up, when relay HD is picked up in response to a first or clear traflic condition, alternating current of afirst predetermined code rate of a predetermined phase modulation is supplied to the'rails of 'the fsect'ion and when relay HD;

9, contact I80 is released in responseto asecond or approach traffic'condition, alternating current of a second predetermined code rate and said predetermined phase modulation issupplied to the rails.

A trackrelay. CFis connected across the rails at entrance end E of. section D---E,v and when the sectionis unoccupied, that is, when a train shown conventionally at TN does not occupy the section, relay CF is'energized and picked up by the. alternating current supplied to the rails bywthe transmitting. apparatus located .at the exit end D of the section. As here shown the relay CF is effectively energized and picked up whenxthelsection is unoccupied irrespective of the coding of the: alternating current and is dc.-

energized and releasedwhen the track section is occupied. Relay CF wouldbe used. to control apparatus associated with .the section next in the rear of sectionD-E, and which apparatus would includena.trafiic controlled relay corresponding to'relayHD for section. DE and controlled by traffic conditions of the section next inadvance. The apparatus governed by relay ,CF.,is not shown for the sake of simplicity, since it would be in. accordance with standardpractice and forms nopart of my invention.

The train .shownconventionally at TN is provided with. train'carried receiving apparatus responsive to, the phase modulated alternating current supplied to the rails of section DE by the transmittingapparatus associated with that section as explained hereinbefore, and which train carried apparatus embodies my invention. In general such train carried receiving .apparatus includes a receiving means, an amplifierdemodulatingmeans, a code following relay, a decoding unit and a signaling means together with the necessary current sources. The receiving means includes inductors I1 and I8 which are mounted on train TN ahead of the leading pair of wheels over rails Ia and lb, respectively, so that eachv inductor picks up an electromotive force corresponding in frequency, phase modulation and coding to that of. the alternating current supplied .to the rails by the transmitting apparatus...

In the present embodiment of. my invention they use of two receiving channels is contemplated and each inductor I1 and I8 is connected tothe input side of an amplifying electron tube, inductor I1 vbeing associated .with a tube VTI and inductor I8 being associated with a tube VT2. Specifically, inductor I1 is connected to primary winding .I9 ofa transformer TI over a condenser and secondary winding 2| of transformer TI and. a condenser 22 in multiple are connected across grid 23 and cathode 24 of tube VTI, a battery 25 being included in the connection .to cathode 24 to provide a desired grid bias voltage. Inductor I0 is connected to primary winding 26 of a transformer T2 over a condenser 21; and secondary winding 28 of transformer T2 and a condenser [9 in multiple are connected across grid 30 and cathode 3| of tube VT2, a battery 32 being included in the connection adjacent cathode 3| to provide a desired grid bias voltage. Tube VTI' is provided with a plate circuit that includes positive termim1 B300 of a suitablesource of direct current not shown, primary winding 33 of a transformer mary winding 35 of. a transformer T4, plate 36 of tube VT2, intervening tube. space to cathode SI and terminal N300.

Electron tubes VT! and VTZ, as well as other electron tubes of the receiving apparatus to be referred to later, are shown as of the indirect heater type although other types of tubes may be used if desired. The filaments of the several tubes are normally heated in the usual manner, such filament circuits being omitted from the drawing for the sake of simplicity. Consequently, the electromotive force picked up .by inductor I7 is amplified by tube VT! and the electromotive force picked up by inductor I8 is amplifier at tube VTZ, the amplified electromotive force appearing in the plate circuit of each of. the two tubes having a frequency, phase modulation and coderate corresponding to that of the rail current. Furthermore, the two amplified electromotive forces caused by such coded rail current are of a predetermined phase relation with respect to each other, because such coded rail current flows in the two track rails in opposite directions at any given instant.

Electromotive forces picked up by inductors l1 and I8 due to stray magnetic fields or magnetized rail spots, and which magnetic conditions ordinarily at least are of the same direction for both inductors or which excite one inductor only or both inductors out of step with .each other, create electromotive forces in the plate circuits of the amplifier tubes that do-not have the predetermined phase relationship with respect to each. other or do not occur simultaneously.

The means of the train carried receiving apparatus for demodulating and detecting the phase modulated energy received by inductors i! and I8 and amplified by the respective tubes VT 1 and VT2, may take any one of several forms. The February, 1939 issue of the Proceedings of the Institute of Radio Engineers includes an article entitled Communication by phase modulation and wherein there are described four different means for receiving phase modulated energy, and it is clear from such article that any .one of these four methods may be used with the train carried receiving apparatus of my invention. In the present embodiment of my invention, demodulation is contemplated by detecting each side band in combination with the carrier separately, and which method is the third listed method of the above cited publication.

In amplitude modulation the two side bands combine with the carrier in such a way that as the side band vectors rotate they add and subtract from the carrier and vary its amplitude, and detection can be accomplished by rectification of the carrier voltage. In phase modulation, the side band vectors rotate to shift the phase of the car- .rier but do not change its amplitude. However, one side band above would combine with the car- -rier to change its amplitude as the side band vec tor rotates, but the envelope of the modulated carrier would contain av strong second harmonic especially if the percentageof modulation is high. Hence, demodulation of the phase modulated energy can be effected by the use of two filters, one of'which passes the carrier and the upper side band frequency and the other of which passes the carrier and the lower side band frequency. Each filter feeds a conventional detector tube, and the output voltages of these detector tubes, are combined in a' push-pull .circuit. This push-pull arangement combines the detector outputs in phase to cancel the second and other even harmonies and leave the fundamental and all odd harmonics with the result that there is created in the output of such detector tubes an electromotive force that is varied at a rate corresponding to the rate at which the carrier is phase modulated.

Since two receiving channels are used, one for the electromotive force picked up from rail la, and one for the electromotive force picked up from rail lb, two such demodulating filters and detector tubes are coupled to the output of each of the amplifying tubes VT l and VT2. Again referring to the drawing, transformer T3 whose primary winding 33 is included in the plate circuit of tube VT], is provided with two secondary windings 3! and 38, winding 31 being connected to an upper side band filter AF! and winding 38 being connected to a lower side band filter BFI. Likewise, transformer T4 Whose primary winding 35 is included in the plate circuit of tube VT2 is provided with two secondary windings 39 and ii), winding 39 being connected to an upper side band filter AF2 and winding 48 being connected to a lower side band filter BFZ. These filters AFI, AF2, BFi and BF2 are preferably similar in their arrangements and each consists of inductance and capacitance as will be readily understood by an inspection of the drawing. 'Each filter AFi and AFZ is proportioned and adjusted to pass the carrier and upper side band and to suppress other frequencies, and each filter BFI and BFZ is proportioned and adjusted to pass the carrier and lower side band and suppress other frequencies.

Filter AFI includes primary winding 6| of a transformer T5 whose secondary winding 42 is connected across grid 43 and cathode M of a detector tube VT3, a biasing battery 65 being preferably included in the connection. Filter BF! includes primary winding 46 of a transformer T8 whose secondary winding 47 is connected across grid 48 and cathode 49 of a detector tube VT l, biasing battery 50 being included in the connection. Detector tubes VT3 and VT& are provided with a push-pull arrangement of plate circuits, one primary winding 5| of a transformer T9 being included in the plate circuit of tube VT.3 and another primary winding 52 of transformer T9 being included in the plate circuit of tube VT4. Both of these plate circuits are connected to a common source of current, such as, the B30fi NEGEJ source of direct current. I

Filter AFZ includes a primary winding 53 of a transformer T1 whose secondary winding 54 is connected across grid 55 and cathode 5E5 of a detector tube VT5, a biasing battery 51 being included in the connection; and filter BF2 includes primary winding 58 of a transformer T8 whose secondary winding 59 is connected across grid 60 and cathode 5| of a detectortube VTB, a biasing battery 62 being included in the grid circuit. Tubes VT5 and VTE are provided with a pushpull arrangement of plate circuits, one primary winding 63 of a transformer TH) being included in the plate circuit of tube VT5 and another primary winding 64 of transformer TH) being included in the plate circuit of tube VTS, and the two plate circuits being connected to the B300- N33B current source.

It follows that each change from non-modulated energy to modulated energy and from modulated energy back to non-modulated energy of the rail current creates a variation in the output of each of the two demodulators. In other words, an electromotive force is created in secondary winding 65 of transformer T9 of the demodulator including filters AFI and BFI, and tubes VT3 and VT4, as the result of the phase modulated energy picked up by inductor l1, and a similar electromotive force is created in secondary winding 66 of transformer THI of the demodulator including filters AF2 and BF2, and tubes VT5 and VT6, as the result of the phase modulated energy picked up by inductor l8.

The electromotive forces of secondary windings 65 and 66 of transformers T9 and TI 0, respectively, are used to operate a code following relay MR through the means of a mixer tube VT] and a transformer Tl l. Secondary winding 65 of transformer T9 is connected across a first grid 61 and cathode 68 of tube W1, and secondary winding 66 of transformer Till is connected across a second grid 69 and cathode 68 of, tube VTl. A biasing battery 10 is common to the two grid circuits of tube VTT to provide a negative grid bias voltage for tube VT'I such that plate circuit current flows only when the electromotive forces of secondary windings 65 and 66 are applied to the grids 61 and 69 simultaneously and in phase.

Tube VTI is provided with a plate circuit including the BMW-N306 current source and primary winding ll of transformer Tl I, the secondary winding 12 of transformer Tl I being connected to the code following relay MR. As stated above, the arrangement is such that when the two electromotive forces created in secondary windings 65 and 66 are applied to tube VT! simultaneously and in phase a plate circuit current flows and consequently an electromotive force is induced in secondary winding I2 of transformer TH and an energizing current impulse is in turn applied to relay MR for operating that relay. It follows that the code following relay MR is operated at a rate corresponding to the code rate at which the rail current is phase modulated.

Code following relay MR controls two control relays A and L through a decoding unit DU. This decoding unit DU may be of any one of several well-known forms and is shown conventionally for the sake of simplicity since its specific structure forms no part of my invention. For example, decoding unit DU may be of the standard tuned .circuitarrangement, and it is sufficient for this application topoint out that when relay MR is operated at a rate correspond ing to the 180 code rate of phase modulation of a the rail current, control relays A and L are both picked up. and when relay MR is operated at a rate correspondingto the '75 code rate of phase modulation of the railcurrent. relay L is picked up and relay A is released. Also. relays A and L are both released when the code. following relay MR is inactive.

Control relays A and L are used to control in turn a train control device such as a multiple osition cab signal CS. The arrangement is such thatwhen relay A ispicked up to close front contact 13. a ircuit is formed for a green lamp G of. signal CS and lamp G is illuminated as a clear signal indication. When the relay A is released closing back contact 14 and relay L is picked up to close frontcontact 15. a circuit is formed for yellow lamp Y of signal CS and lamp Y is illuminated as an approach signal indication: and when relays A and L are released closin back contacts M and 6. respectively. a. circuit is formed for red lamp R of signal CS and the operation of the train carried receiving ap paratus is the same as described in connectionlamp R is illuminated as a slow speed signal indication. a l

Indescribing the operation of the apparatus I shall assume that train TN occupies track section DE at a time when control relay HD is picked up due to clear traflic conditions and the alternating current supplied to the rails at exit end D is phase modulated at the coderate of'180. Such coded rail current flows in the rails in opposite directions at any given instant and induces electromotive forces in each'inductor l1 and I8 and each of which electromotiveforces is phase modulated at a code rate corresponding to that of the rail current. The electromotive force picked up by inductor ll is applied to the receiv-- ing channel including amplifier tube VTl and-the demodulator comprising filters AFI and BFl and detector tubes VT3 and VT4, and bywhich receiving channel such electromotive force isamplified and demodulated causing an electromotive force having a code rate that corresponds to the code rate of the phase modulation of the electromotive force picked .up' from rail id, to be created in secondary winding 65 of the transformer T9. The electromotive force picked up by inductor i8 is applied to the receiving channel in cluding amplifier tubeVT! and the demodulator comprising filters AF2 and BF2 and detector tubes VT5 and VT6, and by which receiving channel such electromotive force is amplified and demodulated to causein secondary winding 66 of transformer TIU an electromotive force having a code rate that corresponds to the code rate of the phase modulation of the electromotive force picked up from rail lb. Thesetwo output electromotive forces created in secondary wind ings 65 and 66 are applied simultaneously to the mixer tube V'Il and as a result an energizing current impulse is applied to the'code following relay MR, relay MR being therefore operated at a rate which corresponds to the coderate of the phase modulation of the rail current. Such rate of operation of code following relay MR efi'ectively energizes relay A and'relay A on picking up causes lamp G of signal CS to be illumi nated to display a clear signal indication.

In the event train TN occupies section D-r-E when relay I-ID is released in response to approach traffic conditions and the alternating our rent isphase modulated at the code rate of '75,

with the 180 code rate for the phase modulation of the rail current except for the fact of the code rate of the electromotive forces created in secondary windin s 65 and 66 are of the 75' code rate and relay MR is operated at a correspond: ing rate. With code following-relay MR operated at a rate corresponding to'the 75 code rate; relay L ispicked up and relay A is released and lamp Y of signal CS is illuminated as an approach signal indication.

In the event a train' occupies section D E ahead of the train TN and thereby shunts the rail current away from train TN, no energy is picked up by inductors l1 and I8 and relay MR is inactive, with the result that both relays A and L are released and lamp R of signal C Sis lluminated, to display a slow speed indication; It is to be pointed out that if'the'energy level of th electromotive forces picked'up by inductors. IT and i8 is relatively high/the amplifyin tubes VTI and VT2 may be omitted and inductors l1 and! connected directly to the corremondin demodulators.

Again, it is apparent that inductors IT and t8 1 eluding may be connected together and the two inductors connected to a single receiving channel, such as the channel including tube VT! and the demodulator comprising filters AF! and BFI, and the detector tubes VT3 and VT4. 'Under such condi tions the mixer tube VT1 would be provided with one control grid and that grid connected to the output of the demodulator and biased so that a corresponding impulse of current flows in the plate circuit of tube VT1 whenever an electromotive force is applied to its control grid.

It is also to be noted'that the particular phase shift used in the phase modulation is not critical. However, the effective amplitude of the modulation frequency increases as the phase shift increases, and the resulting increase in the third harmonic would not disturb the operation of the code following relay. A mathematical analysis showsthat a phase shift of the order of 100 degrees would produce about 1.7 times the power saving obtained by a phase shift of one radian, and 180 degrees phase shift would show a power saving of about 1.86. It is to be observed that if a phase shift of 180 degrees is used, the coder CT could be arranged with pole changing contacts instead of the make-andbreak contacts and unit PM omitted so that the relative polarity of the track current would be reversed at each operation of the coder.

Advantages of the apparatus here disclosed for a signal system results in more power bein applied to the control or transmitting circuit for a given transmitter. Such power gain for a cab signal system results, in the particular advantages of operating a cab signal over track sections of 11,000 feet of length on a carrier frequency which provides satisfactory broken rail protection; Such power gain results in a reduction of the interference created by stray magnetic fields and magnetized rail spots. Also a two. channel arrangement for the receiving apparatus reduces the interference of stray magnetic fields and has the additional advantage that satisfactory broken rail protection is obtained for carrier frequencies of the order of 40 cycles per second when used in track circuits of 11,000 feet in length.

Although, I have herein-shown and described only one formof apparatus embodying my invention, it. is understood that various changes and modifications may be made therein within the scope of theappended claims without departin from. the spirit and scope of myinvention.

'Having thus described my invention, what I claim is: I

1. In combination, a control circuit, a source of alternating current, a code transmitter opera tive to periodically operate a contact member to alternately engagea first and a second contact at a predetermined code rate, a modulating-unit ininductance and capacitance, circuit means including two alternative paths to connect said current source to sa d control circuit to supply eurrent thereto, one of said paths including said contact member and first contact to supply to said control circuit such alternatingcurrent non-modulated and the other of said paths including said modulating unit in series with said contact member and second contact to impress on the current supplied to the control circuit a phase modulation predetermined in phase shift by said modulating unit, a demodulator operative to create current impulses in response to such phase modulation of such alternating current, d'ecoding means peculiarly responsive to currentimpulses of said'code rate, a signal connected to of alternating current, a modulating unit includ ing reactance and having input-terminals connected to said current source, a code contact member operable to alternately engage a first and a second contact at preselected coderate, a first circuit path including said contact member and said first contact to connect said current source to said control circuit, a second circuit path -in-- cluding said con-tact member and sai second contact to connect the output terminals of saidmodulating unit to the control circuit, said first and second circuit paths effective to cause the.

current supplied from said source to the control.

circuit to be characterized by a predetermined code rate of phase modulation; and receiving apparatus including a phase demodulator, a decoderv and a signal and connected to the control circuit to actuate said signal in response to the current thus supplied to the control circuit.

3. In combination, a control circuit; transmit;

ting apparatus including a source of alternating current, a phase modulator including inductance and capacitance, a coding device having a contact member alternately operated to a first and .a sec: end position, said current source alternately con nected to said control circuit through the first position of said contact member and through the second position of the contact member in series with said modulator to supply thereto such alters nating current characterized by alternate periods of current non-modulated and periods of current modulated to have a predetermined phase shift; two inductors coupled to said control circuit to pick up two electromotive forces .each characterized by the code rate of phase modulation of the control circuit current, two demodulators one connected to each inductor and each said demodulator operative to creat current impulses of said code rate in response to the electromotive force pickedup by the respective inductor, decoding means. peculiarly operative in response to current impulses of said code rate and connected to said demodulators in multipl for operation thereof onlywhen the current impulses created by the demodulators occur simultaneously, and .a signal governed by saiddecoding means.

4. In combination; transmitting apparatus in: cluding a source of alternating current, a phase modulator having inductance and capacitance and a coding device having a contact member operable to a first and a second position to alter: nately connect, said modulator in series with said sourc and shunt said source around the modulator to supply'such alternating current charac terized by a preselected cod rate of a predetermined phase modulation; a control circuit including two adjacent conductors to form opposite sides of said circuit and ,saidcontrol circuit connected to said transmitting apparatus to be supplied by said coded and modulated alternating current, two inductors one coupled ,to each of said conductors to pick up two electromotive forces each characterized by said cod rate of said phase modulation and which ele'ctromotive forces have a predetermined relative polarity with respect to each other, two demodulators one con} nected to each of said inductors and each demodulator efiective to create current'impulses of a rate corresponding to saidcode rate of said phase modulation in response to the electromotive force picked up by the respective inductor, a mixer electron tube provided with two grids, said two grids connected one to one of said demodulators and the other to the other one of said demodulators to creat a current impulse in the plate circuit of said mixer tube only when two current impulses one from each demodulator are applied to said grids simultaneously and at a preselected relative polarity, and signaling means coupled to said mixer tube plate circuit effectively controlled by the code rate of current impulses thus caused to flow in said plate circuit.

5. In combination, a source of alternating current, a phase modulator including inductance and capacitance to create a predetermined phase shift of the current from said source, a coder including a contact member operated to alternately engage a first and a second contact at a preselected code rate, a control circuit having adjacent conductors as opposite sides of the circuit, circuit means including alternative paths to connect said source across said control circuit conductors, one of said paths including said first contact and the other of said paths including said modulator and said second contact to cause the current supplied to said control circuit to be of substantially a given amplitude but havin recurrent periods during which the current has said predetermined phase modulation, two inductors one coupled to each of said conductors to receive two electromotive forces each having a phase modulation corresponding to that of the current in the respective conductor, two phase demodulators one connected to each of said inductors to create two groups of current impulses each group having a code rate corresponding to the code rate of the phase modulation of the electromotive force of the respective inductor, a mixer tube provided two grids one of which is connected to one of said demodulators and the other to the other of said demodulators to create a predetermined variation in the plate current of the mixer tube when its two grids are simultaneously excited by the two groups of current impulses, a relay coupled to said plate circuit and operated in response to such variations of plate current, and signaling means controlled by said relay and effectively energized only when said relay is operated at a rate corresponding to said code rate.

WILLARD P. PLACE. 

